I prefer to have all the shirt to be in the same direction with the designs readable from the bottom of the bed. When I have different sizes of t-shirt I measure the larges design and an inch or two to the measurements top and bottom and then I make all my blocks that size. Sometimes I will add two small one with sashing in between and even on the sides to make a block that is the same as the larger ones.
I cut filler pieces out of t-shirt fabric I iron stabilizer on some of the backs of the shirts to extends the blocks to the size I need.